Fifth wheels are used to connect truck tractors and trailers in a way that allows for relative movement between the truck tractor and trailer. Fifth wheels allow the truck tractors and trailers to pivot about a vertical axis when the vehicle combination is negotiating a curve or turn. They also allow for pivoting about a horizontal axis when the vehicle combination encounters rough or uneven terrain.
Fifth wheels are affixed to truck tractors and kingpins are affixed to the trailers. To connect a truck tractor and trailer, the driver backs the truck tractor under the trailer while guiding the kingpin into a gap in the fifth wheel. The driver backs the truck tractor until the kingpin reaches a blind throat or draft port at the end of the gap in the fifth wheel, where a locking mechanism locks onto the kingpin.
To accept the kingpin, the locking mechanism must first be released. To release the locking mechanism to accept the kingpin, the fifth wheel usually includes a manual operating rod. A typical operating rod has a free end accessible from outside the fifth wheel. The free end of an operating rod is usually pulled outwardly to release the locking mechanism to accept the kingpin. The operating rod free end may also be pulled to release the locking mechanism to free a locked kingpin from the locking mechanism.
The fifth wheel is usually mounted on a pair of bracket bases affixed to the truck tractor. Trunnion pins are usually used to secure this mounting. Typical trunnion pins have common horizontal axes so that the fifth wheel may pivot about the horizontal axes as the vehicle travels over uneven terrain. The fifth wheel may also pivot on the trunnion pins when coupling the truck tractor and trailer. While they may rotate and the fifth wheel plate may rotate on them, the trunnion pins must not move outwardly along their axes or the fifth wheel plate could become dislodged from the truck tractor. To retain the trunnion pins in position, a retainer device like a cotter pin, clinch pin, through bolt, or the like is typically used. Usually, retainers extend through diametric holes at the interior ends of the trunnion pins. To insert or remove the retainer, the installer must reach under the fifth wheel plate and work without easy access to the retainer or diametric hole.
When backing the truck tractor under the trailer, it sometimes happens that the driver misses the kingpin with the mouth or gap of the fifth wheel plate. In such a case, the misfed kingpin may travel around the outer edge of the fifth wheel plate. In traveling around the fifth wheel plate edge, the misfed kingpin may hit and damage the extended operating rod if the driver continues to back up. Or the misfed kingpin may travel over the top surface of the fifth wheel plate and slide over the front part of the edge of the fifth wheel plate. The driver may mistakenly believe that the misfed kingpin is in place and begin to pull forward; the misfed kingpin that slid over the front edge may travel around the outer edge of the fifth wheel plate. It may hit the extended operating rod, damaging it and requiring premature replacement of the operating rod, the operating mechanism components, or both.
The problem of a misfed kingpin may commonly arise in "yard spotting." In yard spotting, a driver moves trailers from place to place within a yard without raising the landing gear. The yard tractor usually has a fifth wheel mounted on a hydraulic boom. The frictional connection between the fifth wheel plate and trailer bed plate may give the driver the impression that the trailer and yard tractor are coupled when the kingpin has actually been misfed and misdirected. Such a situation could be dangerous if the trailer slips off the fifth wheel plate.